The present invention provides stabilised photosensitive materials and methods for their preparation.
Many drugs and other active agents undergo photodegradation on exposure to UV light and this can be reduced to some extent by combining them with light absorbing agents. Known photostable compounds are capable of acting as light absorbing agents and are used in sunscreens to protect against damage to the skin by radiation, particularly UVA (320-400 nm) and UVB (290-320 nm) radiation, UVB being the most harmful to the skin. Protection is brought about by the absorption of the relevant photons by the chromophore in the sunscreen agent. Alternatively photosensitive materials can be incorporated into cyclodextrins, eg by forming inclusion complexes as described in Andersen F and Bundgarrd H (1984) Int J Pharm 19: pp 189-192.
While such methods are adequate for some purposes improved methods of increasing photostability are desirable, particularly in the area of pharmaceutical medicine where agents must sometimes be administered in their less stable form. In particular a method of increasing photostability would be desirable to lengthen the shelf life of a photosensitive material or its effect when applied locally.
According to this invention a composition comprises a material retained within a liposome, characterised in that the composition further comprises an agent capable of increasing photostabilisation of the composition.
The material to be stabilised by use of the compositions of the present invention is any that may suitably be incorporated into the liposome. This method is particularly useful for the stabilisation of photosensitive materials such as pharmaceuticals or vitamins, eg riboflavin, riboflavin-5xe2x80x2-(dihydrogen-phosphate) or a salt thereof. Organic material, eg organisms may also be photosensitive materials which could be photostabilised by incorporation into liposomes of the present invention.
The liposome may suitably be multilamellar, eg liposomes prepared by the dehydration-rehydration technique, DRVs. These have the advantage of having high yield entrapment values, up to 80% or more. Giant liposomes such as unilamellar giant liposomes capable of entrapping live bacteria may also be utilised in the performance of the present invention. One method of producing such liposomes is described in PCT Patent Application Number WO 95/09610. It is expected that multilamellar liposomes also comprise a number of unilamellar liposomes.
The agent may suitably be associated with the membrane of the liposome, with either the aqueous or lipid phase. Preferred embodiments are those in which the agent is associated with the lipid phase of the membrane but alternatively more than one agent may be included and these may be associated with the same or different phases of the liposome.
The liposomes may be formed from any suitable lipid components, with optional additional components, eg sterols such as cholesterols. Cholesterol enhances the stability of liposomes by inhibiting the movement of acylated chains and increasing the thickness of the bilayers.
According to one aspect of the invention the agent comprises a light absorbing material. The light absorbing material may suitably be any material capable of absorbing some or all visible and/or ultraviolet (UV) light. Preferred light absorbers are those that absorb UV.
Light absorbing materials include chemical absorbers and physical blockers of UV radiation. Chemical absorbers are generally aromatic compounds conjugated with a carbonyl group. In many cases an electron-releasing group (an amine or methoxyl group) is substituted in the ortho- or para- position of the aromatic ring. Chemicals of this configuration absorb the harmful shortwave (high energy) UV rays (250-340 nm) and convert the remaining energy into innocuous longer wave (low energy) radiation (usually above 380 nm). They are excited from their ground state to a higher energy state by the absorption of the UV radiation. As the excited molecule returns to its ground state energy is emitted which is lower in magnitude (longer wavelengths) than the energy initially absorbed to cause the excitation.
Chemical absorbers are characterised not only by their chemical properties but also by their solubility in hydrophobic (oil Red 0, oxybenzone, deoxybenzone) and hydrophilic (sulisobenzone) substances as well. Physical blockers reflect or scatter the ultraviolet, visible and infrared rays and usually include metal oxides. Antioxidants such as beta carotene quench both singlet oxygen driven photochemical reactions and free radical reactions.
Examples of suitable light absorbing materials include azo compounds such as oil Red 0 (solvent red 27; CI26125; 1-[{4-[xylylazo]xylyl}-azo]-2-naphthol), and benzones such as sulisobenzone (5-benzoyl-4-hydroxy-2-methoxy-benzenesulphonic acid), oxybenzone (2-hydroxy-4-methoxy-benzophenone) and deoxybenzone (4-methoxyphenone). Other suitable light absorbing materials will occur to those skilled in the art.
According to another aspect of the present invention the agent comprises a compound capable of forming an inclusion complex with the photosensitive material. Any compound capable of forming a suitable inclusion complex with the photosensitive material may be used. An example of such suitable compounds are the cyclodextrins, these being cyclic oligosaccharides composed of at least six xcex11xe2x86x924 linked D-glucose units with a cavity of fixed size and shape which are capable of forming an inclusion complexes with a variety of materials. The term cyclodextrins will be understood to include cyclodextrins and their derivatives, eg ether, ester and amide derivatives. Suitable cyclodextrins include xcex2-cyclodextrin, gamma cyclodextrin, hydroxy-propyl-cyclodextrin, methyl-xcex2-cyclodextrin and polymer-xcex2-cyclodextrin.
One embodiment of the present invention comprises sufficient cyclodextrin to form an inclusion complex comprising xcex2-cyclodextrin or hydroxypropyl-cyclodextrin in molar ratio photosensitive material:cyclodextrin of 1:1. A further embodiment comprises sufficient cyclodextrin to form an inclusion complex comprising gamma-cyclodextrin, hydroxypropyl-cyclodextrin and polymer-xcex2-cyclodextrin in molar ratio photosensitive compound:cyclodextrin of 1:2 or 1:1. A still further embodiment comprises polymer-xcex2-cyclodextrin with molecular weight between 4000 and 4500 as the agent capable of forming an inclusion complex with the photosensitive material.
According to a further aspect of the invention the agent comprises an antioxidant. Suitable antioxidants will be apparent to the person skilled in the art. One such suitable agent is beta-carotene.
It will be understood that the above aspects of the invention are not mutually exclusive. The invention also provides embodiments in which two or more agents are included, selected from light absorbing agents, compounds capable of forming inclusion compounds and antioxidants.
A preferred embodiment of the invention combines the agents described in the first three aspects of the invention in order to express simultaneously and dynamically their protective effect. This may be achieved by using multilamellar vesicles comprising a hydrophilic cyclodextrin capable of forming an inclusion complex with the photosensitive material in the aqueous phase and a combination of light absorbing and antioxidant agents in the lipid bilayer.
The present invention also provides methods of producing a composition as described above comprising the incorporation into a liposome of an agent capable of effecting the photostabilisation of the composition; in particular methods wherein the liposome is prepared by a dehydration rehydration technique.
One embodiment of this aspect of the invention provides a method comprising:
(a) mixing the components from which the liposome is to be formed in the presence of an organic solvent;
(b) evaporating the solvent;
(c) dispersing the film formed in (b) in water and isolating the liposomes so produced,
characterised in that an agent capable of effecting photostabilisation is added in step (a) and/or (c) and the photosensitive material is added in step (c) or on further rehydration after dehydration.
Any suitable organic solvent may be used in step (a), eg chloroform. In step (b) the solvent may suitably be evaporated by rotary evaporation. Dispersion in step (c) may be performed by any suitable method, a preferred method being sonication, using an ultrasonic probe.
The agent capable of effecting photostabilisation may be added in step (a) if lipid soluble or in step (c) if aqueous soluble. The photosensitive material is added in step (c) or on rehydration after further dehydration. The photosensitive material may be added in aqueous solution or it may be added in the form of an inclusion complex.
The liposomes formed may be freeze dried and the resulting pellets or powder stored until further required when they can be rehydrated.
The present invention further provides pharmaceutical mixtures comprising a composition as described above together with a physiologically acceptable carrier, suitable for administering to a living organism preferably compositions suitable for administering to humans. Furthermore the invention provides pharmaceutical mixtures comprising a composition as described above together with a physiologically acceptable carrier characterised in that it is suitable for administration to a human by injection, orally, inhalation or topical administration.
Suitable physiologically acceptable carriers to be included in such compositions will occur to those skilled in the art, eg saline solutions.
The materials and methods of the invention will now be illustrated by example only with reference to the following non-limiting examples. Further embodiments will occur to those skilled in the art in the light of these.